The present invention relates to an electromechanical actuator, and particularly a combination of an electric motor and a control and drive circuit for controlling the operation of the motor itself.
FIG. 1 shows an arrangement including a stepping motor 1, an example of an electric motor, and a drive circuit 2 therefor. The drive circuit 2 comprises power transistors for switching on and off the electric current to the stator windings of the stepping motor 1. A timing control circuit 3 provides the drive circuit 2 with timing control signals to control the timing of the operation of the stepping motor 1. A controller 4 is formed of a microprocessor (CPU) or the like and provides the timing control circuit 3 with control signals. The drive currents to the stepping motor 1 are made to flow through output lines 5. Drive signal lines 6 and control signal lines 7 are also provided. V.sub.P and E.sub.P denote the power supply and the ground for the drive circuit 2. V.sub.L and E.sub.L denote the power supply and the ground for the timing control circuit 3.
The controller 4 provides control signals of a specific mode commanding the direction of rotation, i.e., forward or reverse, and the mode of excitation, i.e., single phase, two phase or signal/two phase, and the number of steps of the movement, to control the timing control circuit 3 which follows a specific procedure. The timing control circuit 3 is responsive to the commands from the controller 4 and determines the phase to be excited, the driving time, and the over-drive time, in accordance with a preprogrammed drive procedure. The timing control circuit 3 continues to supply the drive signals along drive signal lines 6 to the drive circuit 2 until the completion of the movement of the commanded number of steps. To maintain the current fed to the stepping motor 1 constant, a constant-current feedback is provided to control the on/off of the drive signals 6. The drive circuit 2 is responsive to the drive signals 6 and turns on and off power transistors provided therein to turn on and off the drive currents to each phase of the stepping motor 1. When an overdrive circuit is used, the current reference is switched to a larger value thereby producing a larger torque.
The stepping motor 1 generates torques by the interaction of the vector sum of the magnetic fields generated by the drive currents to respective phases and the magnetic fields of the rotor magnets, and the motor 1 accordingly rotates and stops. The stepping motor 1 rotates for the number of steps commanded by the control signals from the controller 4.
In a conventional arrangement, the controller 4, the timing control circuit 3 and the drive circuit 2 are mounted on a board separate from the stepping motor 1. Connecting wires or wiring boards are used for conducting the drive currents from the board on which the timing control and drive circuits 2 and 3 are mounted, to the stepping motor 1.
The aforementioned prior art arrangement for driving a motor has the following drawbacks:
(i) The board for the drive circuit and other components is disposed externally to the stepping motor, and therefore a large number of conductors are required between the board and the stepping motor. This forms an obstacle to reduction in size and reduction in cost.
(ii) The conductors 5 between the board and the stepping motor can create interfering electromagnetic fields.
(iii) When the stepping motor is driven with a high power, a heat sink must be provided on each wiring board for heat dissipation. This also forms an obstacle to size reduction and cost reduction.
(iv) When a number of stepping motors must be driven, separate conductors are needed for each of the motors. The total number of conductors can therefore be very large.